How can I color a text?
Mesh can't use a material which is not contained in slots.

I will try to guess what you have done so far. You create a text, make it thick enough, turn it into mesh. And for that wall, you use a bool modifier with difference operation, select the mesh text. And get no color?

1) Make sure your text have an material on it



2) In your wall object, you will need a same material in the slot



At the time you add the same material on the wall, you should see the text color in your wall.